In recent years due to the constant increase of energy prices dictated, in particular, by the constant increase in the price of fossil fuels, more and more interest has been shown towards renewable energy sources. One particularly appreciated form of renewable energy source consists in sunlight converted into electricity by means of photovoltaic generators.
The advantage of a photovoltaic generator over other renewable energy sources such as wind, waves and biomasses includes the fact that it does not require moving pieces, thereby, lowering costs and maintenance, and that can be installed virtually anywhere in sizes ranging from a few square centimeters to photovoltaic parks, covering several square kilometers.
One particularly advantageous form of photovoltaic generators consists of concentrated photovoltaic generators. Even more specifically, concentrated photovoltaic generators are preferred since they reduce material costs compared to the non-concentrated photovoltaic generators. In particular, the sunlight is concentrated via at least one lens onto a cell. In this manner, the size of the cell can be reduced, thereby, lowering the costs associated with semiconductor materials and processes necessary for realizing the cell.
Usually, in concentrated photovoltaic generators, Fresnel lenses are used. The advantage of Fresnel lenses over a standard lens consists in the fact that they can achieve the same value of sunlight concentration with a thinner surface, thereby, lowering space requirements and cost of materials for the lenses. In most of the cases, the Fresnel lenses are square, or rectangular, and arranged as a lens plate with several lenses next to each other. For instance, FIG. 4 schematically illustrates a top view of two lenses 4100 and 4200 realizing a lens plate 4000. In particular, in lens 4200, a plurality of circular lines illustrates facets of the Fresnel lens. In lens 4100, only some of those facets have been represented for ease of illustration. More specifically, the smallest illustrated facet in lens 4100 corresponds to a radius R41. A bigger facet corresponding to radius R42 is further illustrated together with a facet corresponding to an even bigger radius R43. As can be seen, only facets having a radius which is smaller than or equal to half of the width 4001 of the lens 4100 achieve a complete circumference. This is, for instance, the case for facets having radius R41 and R42. For facets such as the one having radius R43, only parts of the circumference fit within the lens 4100. The last circumference fitting in the lens 4100 is ideally the one corresponding to radius R44 having the biggest value fitting in the lens 4100.
FIG. 1 schematically illustrates a cut view of a single Fresnel lens 1000, taken along the diameter of the lens.
In particular, Fresnel lens 1000 is realized on a substrate 1001, such as glass or plastic, having a plurality of lens sections 1100-1600. Even more specifically, each of the lens sections 1100-1600 comprises a slope facet and a draft facet. For instance, in relation to lens sections 1500, slope facet 1540 and draft facet 1550 are indicated. The slope and draft facets are defined by their angles and lengths. More specifically, the slope facet 1540 of each of the lens sections 1100-1600 has a slope angle 1341 indicated, with reference to respective slope facet 1540 of lens sections 1300. Similarly, the draft facet 1550 of each of the lens sections 1100-1600 has a draft angle 1151 indicated with reference to respective draft facet 1550 of lens section 1100.
The slope facet 1540 is the facet providing the optical function. On the other hand, the draft facet 1550 is used in order to achieve a flat shape for the Fresnel lens 1000, but provides no optical functionality. It is, therefore, generally an object of a Fresnel lens to have a draft angle 1151 as small as possible, in order to reduce the surface of the Fresnel lens not achieving any optical functionality, so as to maximize the efficiency of the lens.
However, reduction of the draft angle 1151 is only possible to a certain degree. This is due in particular to the fabrication method of replicated Fresnel lenses. As machined Fresnel lenses are much too expensive, the much less expensive method of replicating Fresnel lenses is employed. Usually, a first master structure used to realize Fresnel lenses is first manufactured on a diamond turning into a metal substrate. This first master structure is then replicated into nickel by using electroforming. From the first nickel replica, further nickel molds are made in order to build up a complete production tooling which is then used in high volume production of replicated lenses, or lens plates. A critical process step in this fabrication method is the separation of a pair of nickel molds. In this step, the draft angle 1151 plays a critical role because the smaller the angle, the larger the forces which will be required to separate the two nickel molds. This usually implies that the angle cannot be reduced to zero, but an optimum minimum angle has to be found between the optical efficiency of the Fresnel lens and the manufacturability of the lens itself via the process described above.